Which DB should I use?

I am now building an application which should store and handle large amounts of data. So now I'm struggling with the question - which DB should I use.

My requirements are:

• Handle up to ~100,000 insert commands a second (sometimes several ones from different threads). 100,000 is the peak; Most of the time the amount would be between hundreds to a few thousands.
• Store millions of records.
• Query the data as quickly as possible.
• Part of the data properties change for every entity, which fits non-relational database behavior more than relational ones. However, the sum of possible properties is not huge, so it can be presented as columns in a relational database (if it's much faster this way).
• Update commands will rarely occur.

Which DB would you recommend me to use?

Thanks!

Update: The OS I'm using isn't Windows. I thought that if SQL Server would be the most recommended DB then I might switch but from your responses, this is not the case.

Regarding the budget - I will start with the cheapest option and I guess that this will change once the company has more money and more users.

No one has recommended no-sql databases. Are they really that bad for this kind of requirements?